Systems and methods of reversibly controlling the oxygen vacancy concentration and distribution in oxide heterostructures consisting of electronically conducting In2O3 films grown on ionically conducting Y2O3-stabilized ZrO2 substrates. oxygen ion redistribution across the heterointerface is induced using an applied electric field oriented in the plane of the interface, resulting in controlled oxygen vacancy (and hence electron) doping of the film and possible orders-of-magnitude enhancement of the film's electrical conduction. The reversible modified behavior is dependent on interface properties and is attained without cation doping or changes in the gas environment in contact with the sample.
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8. A method of controlling oxygen vacancy distribution in a hetero structure, comprising:
applying an electric field to the heterostructure, the heterostructure having an Y2O3 stabilized ZrO2 substrate and an oxide semiconductor layer with an interface there between, with a first electrode at a first end in contact with both the substrate and the semiconductor layer and a second electrode at a second end in contact with both the substrate and the semiconductor layer;
moving oxygen ions from the oxide semiconductor layer to the substrate;
developing an applied current moving parallel to a longitudinal axis of the interface from the first electrode to the second electrode;
developing a voltage component orthogonal to the applied current; and
altering a property of the oxide semiconductor layer while the electric field is applied.
1. A method of controlling oxygen vacancy distribution in a hetero structure, comprising:
applying an electric field to the heterostructure, the heterostructure having an oxygen ion conductor substrate and an oxide semiconductor layer with an interface there between, with a first electrode at a first end in contact with both the oxygen ion conductor substrate and the semiconductor layer and a second electrode at a second end in contact with both the oxygen ion conductor substrate and the semiconductor layer;
moving oxygen ions from the oxide semiconductor layer to the oxygen ion conductor substrate;
developing an applied current moving parallel to a longitudinal axis of the interface from the first electrode to the second electrode;
developing a voltage component orthogonal to the applied current;
wherein the conductance of the oxide semiconductor layer is increased by application of the electric field.
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This application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/517,809, filed Jun. 9, 2017, the content of which is herein incorporated by reference in its entirety.
The United States Government claims certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and the University of Chicago and/or pursuant to DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.
The present disclosure relates generally to methods relating to thin films and oxygen vacancy control.
The discovery of a wide range of remarkable behaviors in thin film oxide heterostructures has led to intense interest in their synthesis and characterization. Oxygen vacancy concentrations exceeding bulk levels near surfaces and interfaces have recently been cited as instrumental in controlling these behaviors. For example, localized metallic conduction near heterointerfaces between bulk oxide insulators (e.g., LaAlO3/SrTiO3) has been interpreted by several authors to be due to vacancy doping of SrTiO3 substrates during thin film deposition. Large changes in electrical resistance associated with metal-insulator transitions or resistive switching behavior have also been linked to increased oxygen vacancy concentrations in thin film heterostructures. Magnetism in thin film heterostructures of bulk non-magnetic materials has been attributed to the presence of oxygen vacancies near interface and catalytic activity of oxide heterostructures has been explained by enhanced interfacial oxygen vacancy concentration. These examples demonstrate the importance of being able to manipulate oxygen vacancies to achieve emergent behavior, motivating the exploration of new ways to control vacancy concentrations and distributions in oxide thin film heterostructures.
Attempts have been made to control oxygen vacancy concentration by changing O2, but that requires changing the case environment of the system. Other attempts have focused on cation doping, particularly by substituting an aliovalent cation, but those techniques are irreversible. Another attempt has looked at applying potential normal to a surface, but the vacancy concentration only changes when a conducting electrolyte is present.
Embodiments described herein relate generally to systems and methods for dynamic and reversible oxygen vacancy doping and controllable enhanced electronic conduction in oxide heterostructures, without requiring changes in cation composition or the gas environment in contact with the sample.
It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein.
The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several implementations in accordance with the disclosure and are therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.
Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.
Embodiments described herein relate generally to oxygen vacancy doping and thin films. Specifically, methods and systems are described for achieving a dynamic and reversible oxygen vacancy doping and controllable enhanced electronic conduction in oxide heterostructures, without requiring changes in cation composition or the gas environment in contact with the sample. A voltage component in the direction normal to an interface plane is achieved without applying a voltage in that direction, i.e. the direction of the applied field is in the plane of the interface, but a voltage component develops that is normal to the direction of the applied field. Therefore, in one embodiment an equivalent property-changing voltage can be attained without requiring the placement of electrodes on the surfaces “above” and “below” the interface plane. This allows for free design of the space that would be parallel to the surface. As a non-limiting example, devices could be manufactured using the described methods and structure where chemical reactions are taking place on a surface. In the prior art, the reaction would be blocked from proceeding if an electrode, necessary for the voltage component that is “normal” to the surface, was blocking the surface from contacting the reactants. Another non-limiting example, an optical device may use a film with changing optical properties caused by interaction with the film by changing the vacancy content of the film. The prior art would an electrode position on a surface parallel to the interface plane.
One embodiment for controlling oxygen consists of an epitaxial thin film of In2O3 grown on the (001) surface of a Y2O3-stabilized ZrO2 (YSZ) substrate. This thin film is described herein for illustrative purposes as a model heterostructure. In one embodiment, a ionic conductor and an electronically dopable material are provided and have an interface therebeween.
Using the heterostructure design illustrated in
The conductance enhancement was found to be reversible, returning to approximately the original value after removal of the field. The relative increase in conductance was observed to increase with decreasing film thickness (see Table below). This is an indication that the field-induced conduction enhancement is localized to a region in the films near the film/substrate heterointerface. The oxygen partial pressure (pO2) of the sample environment was found to have a small effect on the behavior, with the magnitude of the steady state conduction decreasing with increasing pO2, consistent with n-type behavior, as seen in
Thickness (nm)
4
10
600
16
14
300
70
11
60
The measured conductances of three samples with different thickness In2O3 films were compared at 400° C. when a constant 1V potential was maintained between the voltage pick-off electrodes. Go is the initial measured conductance when the field is applied, G∞ is the field-enhanced conductance at steady state, L is the distance between the pick-off electrodes, and A is the film thickness. The comparison indicates that the enhanced conduction resulting from the field application is highly localized to the In2O3/YSZ heterointerfacial region.
Further experiments were carried out to determine the mechanism responsible for the observed field-induced conductance increases. These measurements clearly demonstrate that the relaxation behavior, showing dramatic enhancement of the sample conduction, is determined by the response of multiple interfaces in the heterostructure to the application of small electric fields. There are five relevant interfaces in the heterostructure. In addition to the YSZ/In2O3 interface 141, electrode/YSZ interface 143a and electrode/In2O3 interface 142a at positive terminals 137, Pt/YSZ interface 142b and Pt/In2O3 interface 143b at negative terminal 131 must be considered. Four of these interfaces play important roles in response to the applied field. The electrode(Pt)/In2O3 interfaces at the voltage pickoffs do not contribute to the observed behavior since a four-electrode configuration is used and pickoff voltages are measured with a high input impedance voltmeter.
Oxygen ions in the ionically conducting YSZ substrate will migrate in an electric field, with oxygen ions attracted to the positive electrode and oxygen vacancies thus moving towards the negative electrode. Oxygen ions enter YSZ at three-phase boundaries at the negative electrode according to O2 (g)+2VO″4e′→2OOx, which is written using Kroger-Vink notation, where VO″ indicates an oxygen vacancy with a +2 charge with respect to that of an oxygen ion, e′ is an electron, and OOx is an oxygen ion with a formal −2 valence sitting in an oxygen lattice site. Oxygen exits the sample at the positive electrode by the reverse reaction.
When a DC voltage is applied across the end terminals (
A Schottky barrier has been reported to develop at the heterointerface between In2O3 films and Pt electrodes when oxygen vacancies are eliminated from the region of the In2O3 film in contact with Pt. In the current experiment, a Schottky barrier will form as oxygen ions migrate towards the positive electrode under the influence of the applied field, eliminating oxygen vacancies from the In2O3 film near the In2O3/positive Pt electrode interface.
As a result of the interfacial resistances described above, a substantial voltage drop develops between the YSZ substrate and the In2O3 film. The voltage drop is concentrated at the interface, where ionic and electronic conductors meet, as shown in
To estimate the extent of the interfacial space-charge region, we used the expression for the Debye length, LD=sqrt[(εKBT)/(v0e2)], where ε and n0 are the dielectric constant and carrier concentration for the undoped (E=0) In2O3 film, KB is the Boltzmann constant, and T is the absolute temperature. Using σ=n0eμ, with σ=10 S/cm and μ=50 cm2/V-sec, we obtain n0=1018/cm3. According to Goswami et al., for In2O3 films, ε is film thickness dependent with a value of about 60ε0 (giving LD=11.3 nm) for d>1500 Å, and varying between 10 ε0 (where LD=4. nm) and 60 ε0 for thinner films. Assuming ε for our films has values in the range of 9-30 ε0 gives an approximate Debye screening length of 4-8 nm.
The development of the potential across the In2O3/YSZ interface was confirmed by directly measuring the voltage between pairs of electrodes on the top and bottom surfaces of samples when a constant voltage was applied between the end electrodes (
As seen in
Further insight into the energetics of oxygen vacancies in In2O3/YSZ heterostructures was obtained from first-principles calculations. The atomic stacking normal to the (001) In2O3/YSZ interface alternates between cation and oxygen planes, with a shared oxygen plane at the interface. As seen in
In contrast to the behavior of In2O3/YSZ heterostructures, we find that heterostructures consisting of In2O3 films grown on MgAl2O4 substrates show no change in conductance in response to application of a DC field, and no change in conductance when the field polarity is reversed (
We note that electric field-induced polarization has been observed to occur in many different mixed oxide conductors (i.e., oxides that exhibit both ionic and electronic conduction behavior), a manifestation of the well-known Hebb-Wagner polarization mechanism. However, the mechanism responsible for the behavior observed in the present study is not consistent with Hebb-Wagner polarization of In2O3. Hebb-Wagner polarization occurs in mixed conductors exposed to a DC field when ionic conduction is blocked by one of the current electrodes. Resultant polarization of the mobile ions may in turn locally alter the electronic conductivity, producing an electronic conductivity gradient. Hebb-Wagner polarization of In2O3 is not expected since In2O3 is not a mixed conductor. In fact, In2O3 has been successfully used as an oxygen-ion-blocking electrode in previous Hebb-Wagner polarization studies, and even when alloyed with ZrO2, the ionic transference number of In2O3 is reported to be only on the order of 10−4. Also, the electrode arrangement used in the present experiment is symmetric (i.e., both the positive and negative current electrodes used in the present case were porous Pt paint, inconsistent with the requirement for Hebb-Wagner polarization that one current electrode block ion transport while the other current electrode allows both ion and electron transport). Most importantly, if Hebb-Wagner polarization of In2O3 were responsible for the field-induced conduction enhancements observed in the present study, we would also expect to see polarization behavior in In2O3 films grown on MgAl2O4 substrates. This is not the case. We also note that, while Hebb-Wagner polarization of YSZ has been reported, the electronic conductance of YSZ is insignificant compared to the magnitude of the measured conductance changes of the samples used in the current study (and the electronic component of the conduction of YSZ is dominantly p-type, in contrast to the observed n-type character of the present samples).
The controllable dynamic doping achieved with application of a small lateral electric field demonstrated in this study should be possible in a wide variety of heterostructures. For example, we have also recently observed similar DC field-induced electrical conduction enhancement in heterostructures consisting of SrTiO3 or WO3 films grown on YSZ substrates (
While we have focused in this study on using oxygen vacancy doping to control electrical conduction behavior in a model thin film heterostructure, we expect that similar heterostructures can be designed and synthesized to control other types of behaviors via oxygen vacancy doping using small electric fields. The approach of using small electric fields to oxygen-vacancy dope heterostructures, which consist of a fast oxygen ion conductor with an electronically conducting functional oxide in a thin film, thus provides a new and unexplored opportunity to tune properties and achieve emergent behavior.
Methods
Epitaxial In2O3 films with thicknesses ranging from 4 to 70 nm were deposited by RF sputtering on single crystal (001) YSZ or MgAl2O4 substrates with dimensions 1×1 cm2×1 mm thickness. The substrate temperature was typically 800° C. and typical growth rates were approximately 10 nm per hour. Sample thickness was characterized using x-ray reflectivity. X-ray diffraction indicated that the films had the expected orientation relation: (001) In2O3∥(001) YSZ, and [100] In2O3∥[100] YSZ.
Samples had parallel line Pt electrodes sputter-deposited through a shadow mask onto the In2O3 film surface and, in some cases, also on the back surface of the substrate. End electrodes were applied using Pt paint. Electrical measurements were made using an environmental probe station, at controlled temperature and pO2. Mass flow controllers were used to mix Ar and O2 gases to achieve desired pO2, at a typical total chamber pressure of 150 Torr. Most measurements were made at pO2=1.5 Torr, at temperatures between 150 and 600° C. DC fields were applied through Pt-coated probe tips in contact with the sample Pt electrodes, using a Keithley Model 2400 source meter. Conduction measurements were typically made in four-point geometry, driving current between the end electrodes and measuring the conductance between a pair of voltage pick-off electrodes on the film surface, under either fixed voltage or fixed current conditions (consistent behavior was observed for both conditions). Some measurements were made using the top electrodes both as the current source/sink electrodes and as voltage pick-offs; similar results were obtained in those cases as when the end electrodes were used as the current source/sink electrodes. Measurements of the voltage across the In2O3/YSZ heterointerface were made by applying a fixed voltage between the end electrodes (
Computational studies used density functional theory within the generalized gradient approximation as implemented in VASP, with Perdew-Wang exchange-correlation functionals and Projected Augmented Wave potentials. Periodic supercell configurations were analyzed, with two interfaces per supercell (each interface is a shared plane of oxygen ions). Two configurations were considered: (1) a flat In2O3 cation plane adjacent to the interface, with In ions occupying two sites (8b and 24d), and (2) a buckled In2O3 cation plane adjacent to the interface, with In atoms only occupying 24d sites. In2O3—ZrO2 supercells contained three layers of ZrO2 and five layers of In2O3 (with 8 cations in each layer). Structural minimization was performed allowing all the atoms and the supercell in the (001) direction to relax, while keeping the other two dimensions fixed at the bulk value. In2O3-YSZ supercells were comprised of five layers of YSZ and five layers of In2O3. Each YSZ cation layer contained 6 Zr ions and 2 Y ions, corresponding to a YSZ stoichiometry equivalent to 6ZrO2—Y2O3. Y ions were distributed in the cell following the approach of a previous computational study.
Embodiments herein described can be utilized with various functions. For example, oxygen vacancy doping can be used to control properties other than electrical conduction, such as chemical reactivity, magnetic properties, or optical properties. Further, particular applications may involve gas sensors, catalysis, separation membranes, superconducting devices, and magnetic devices.
As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a member” is intended to mean a single member or a combination of members, “a material” is intended to mean one or more materials, or a combination thereof.
As used herein, the terms “about” and “approximately” generally mean plus or minus 10% of the stated value. For example, about 0.5 would include 0.45 and 0.55, about 10 would include 9 to 11, about 1000 would include 900 to 1100.
It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples).
The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.
It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Other substitutions, modifications, changes, and omissions may also be made in the design, operating conditions, and arrangement of the various exemplary embodiments without departing from the scope of the present invention.
While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.
Eastman, Jeffrey A., Veal, Boyd W., Zapol, Peter
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